10 Ways to Control Portions: "Learn to 'eyeball' standard portion sizes and stick to them when dining out or dishing up meals.
Keep these tips in mind: 3 oz. of meat is the size of a deck of cards or an audio tape; 1 oz. of meat is the size of a matchbook; 1 cup of potatoes, rice or pasta looks like a tennis ball."

One of the things that fascinates me is the definition of a size of a deck of cards. I am aware of the usual 2x3 inch size of most playing cards, but there are other sorts of card decks out there ... One could be quite correct (if somewhat sneaky) by claiming that the sort of cards (6X8 inches) used by stage-magicians was the deck you were referring to! *attempts to look innocent - fails miserably, but grins mischievously